An insulated conductor such as a wire or a member for a thermocouple is generally applied used in equipment such as heating equipment or fire alarm devices, which require safe operation oat high operating temperatures. Such an insulated wire is also employed in an automobile in an environment which is heated to a high temperature. An insulated wire of this type is generally formed by a conductor which is coated with a heat-resistant organic resin such as polyimide, fluororesin or the like. Such a resin-coated wire can merely withstand a temperature of about 300.degree. C. at the most. However, a wire which is employed in a high vacuum apparatus, for example, must have high heat resistance against baking, etc., a small emission characteristic as to absorbed gas and water for achieving and maintaining a high degree of vacuum, and a small emission of gases caused by thermal decomposition. It is impossible to satisfy such requirements for heat resistance and a non-outgassing property with a conventional wire which is coated with an organic material insulation.
When an insulated wire is used where a high heat resistance is required or in an environment requiring a high degree of vacuum, it is impossible to attain a sufficient heat resistance nor the required non-outgassing property with only an organic coating. In that case, therefore, an insulated wire comprising a conductor which passes through an insulator tube of ceramics, an MI (mineral insulated) cable comprising a conductor which passes through a tube of a heat-resistant alloy, such as stainless steel alloy, that is filled with fine particles of a metal oxide such as magnesium oxide, or the like is generally used.
On the other hand, a glass braided tube insulated wire employing an insulating member of glass fiber fabric or the like is known as an insulated, heat resistant, flexible wire.
Further, wires coated with organic materials were studied. As a result wires have been proposed, one of which is obtained by anodizing an aluminum copper (Cu) or nickel (Ni) conductor for forming an Al oxide layer on the outer wire surface, and another wire is obtained by mixing a frit prepared by mixing various metal oxides with each other and melting and pulverizing the as-obtained mixture for forming a slip, applying this slip to a metal conductor and heating and melting the same for forming a homogeneous composite metal oxide layer or coating on the wire surface.
However, the wire with an aluminum oxide layer is not suitable for use as a heat resistant wire since this technique is merely applicable to an aluminum conductor having a low melting point, while the as-formed film is so porous that the wire has an inferior moisture resistance and a low breakdown voltage.
On the other hand, the wire with a copper nickel oxide coating is applicable to a metal conductor of copper (Cu) or nickel (Ni) having a higher heat resistance. In practice, however, this technique is merely applicable to a metal composite oxide whose melting point is lower by about 300.degree. to 400.degree. C. than those of Cu and Ni since the metal composite oxide layer is formed through a melting process, and the heat resistance temperature is restricted below the just mentioned level. Further, the as-formed wire is inferior in flexibility since it is difficult to reduce the thickness of the film.
In the case of the MI cable, on the other hand, the overall diameter is increased as compared with the conductor diameter, leading to an inferior space factor. Thus, it is impossible to feed a high current.
In the glass braided tube insulated wire, further, fine glass powder is generated and the conductor is disadvantageously exposed due to mesh displacement.